Alzheimer's disease is the most common type of dementia, which accounts for more than half of cases, and the number of patients with this disease is expected to increase more and more in the elderly society in future. As a progressive disease, Alzheimer's disease cannot be fully cured or the progression of the disease cannot be completely blocked by current medical science, but medicines and medical treatments to slow down the progression are available. Thus, it is desirable to detect Alzheimer's disease as early as possible. In particular, since mild cognitive impairment (referred to as MCI) is often developed as a step prior to the onset of Alzheimer's disease, the detection of mild cognitive impairment can help prevent or slow down as much as possible the progression to Alzheimer's disease. Although Alzheimer's disease is currently diagnosed by means of medical interview, MRI scanning of the brain, and the like, it is not an easy task to detect early Alzheimer's disease or mild cognitive impairment.
Thus, to detect Alzheimer's disease at its early stage, methods in which the amount of microRNA (hereinafter referred to as “miRNA”) in blood is used as an indicator are proposed (Patent Documents 1 to 3, Non-Patent Documents 1 to 3).